Stents are widely used for supporting a lumen structure in a patient's body. For example, stents may be used to maintain patency of a coronary artery, carotid artery, cerebral artery, other blood vessels including veins, or other body lumens such as the ureter, urethra, bronchus, esophagus, or other passage.
Stents are commonly metallic tubular structures made from stainless steel, Nitinol, Elgiloy, cobalt chrome alloys, tantalum, and other metals, although polymer stents are known. Stents can be permanent enduring implants, or can be bioabsorbable at least in part. Bioabsorbable stents can be polymeric, bio-polymeric, ceramic, bio-ceramic, or metallic, and may elute over time substances such as drugs. Non-bioabsorbable stents may also release drugs over time. Stents are passed through a body lumen in a collapsed state. At the point of an obstruction or other deployment site in the body lumen, the stent is expanded to an expanded diameter to support the lumen at the deployment site.
In certain designs, stents are open-celled tubes that are expanded by inflatable balloons at the deployment site. This type of stent is often referred to as a “balloon expandable” stent. Stent delivery systems for balloon expandable stents are typically comprised of an inflatable balloon mounted on a two lumen tube. The stent delivery system with stent compressed thereon can be advanced to a treatment site over a guidewire, and the balloon inflated to expand and deploy the stent.
Other stents are so-called “self expanding” stents and do not use balloons to cause the expansion of the stent. An example of a self-expanding stent is a tube (e.g., a coil tube or an open-celled tube) made of an elastically deformable material (e.g., a superelastic material such a nitinol). This type of stent is secured to a stent delivery device under tension in a collapsed state. At the deployment site, the stent is released so that internal tension within the stent causes the stent to self-expand to its enlarged diameter.
Other self-expanding stents are made of so-called shape-memory metals. Such shape-memory stents experience a phase change at the elevated temperature of the human body. The phase change results in expansion from a collapsed state to an enlarged state.
A very popular type of self expanding stent is an open-celled tube made from self-expanding nitinol, for example, the Protégé GPS stent from ev3, Inc. of Plymouth, Minn. Open cell tube stents are commonly made by laser cutting of tubes, or cutting patterns into sheets followed by or preceded by welding the sheet into a tube shape, and other methods. Another delivery technique for a self expanding stent is to mount the collapsed stent on a distal end of a stent delivery system. Such a system can be comprised of an outer tubular member and an inner tubular member. The inner and outer tubular members are axially slideable relative to one another. The stent (in the collapsed state) is mounted surrounding the inner tubular member at its distal end. The outer tubular member (also called the outer sheath) surrounds the stent at the distal end.
Prior to advancing the stent delivery system through the body lumen, a guide wire is first passed through the body lumen to the deployment site. The inner tube of the delivery system is hollow throughout at least a portion of its length such that it can be advanced over the guide wire to the deployment site. The combined structure (i.e., stent mounted on stent delivery system) is passed through the patient's lumen until the distal end of the delivery system arrives at the deployment site within the body lumen. The delivery system and/or the stent may include radiopaque markers to permit a physician to visualize stent positioning under fluoroscopy prior to deployment. At the deployment site, the outer sheath is retracted to expose the stent. The exposed stent is free to self-expand within the body lumen. Following expansion of the stent, the inner tube is free to pass through the stent such that the delivery system can be removed through the body lumen leaving the stent in place at the deployment site.
It can be difficult to estimate the length of the diseased portion of a vessel and therefore the stent length needed for treatment of the disease. This is particularly true for long diseased segments, segments that are tortuous, and segments that are oriented at angles to the plane of the imaging modality used (due to image foreshortening). If the stent chosen for treatment is too long then un-diseased vessel will be treated, and if the stent chosen is too short then diseased vessel will be untreated. Both of these scenarios are undesirable. In some cases physicians will treat a portion of the length of the diseased vessel with a first stent and will implant a second stent to treat the remainder of the length of the diseased vessel, overlapping the two stents to assure that no portion of the diseased vessel is left untreated. This approach is also undesirable because problems such as corrosion between dissimilar metals, excessive vessel stiffening, stent fracture, and reduced stent fatigue life can arise at the site of overlap. Problems secondary to stent fracture can include pain, bleeding, vessel occlusion, vessel perforation, high restenosis rate, non-uniform drug delivery profile, non-even vessel coverage and other problems. Re-intervention may be required to resolve these problems. Further, use of multiple stents to cover a treatment site increases procedural time and cost.
Some have attempted to improve the precision with which to estimate the needed implant length. For example, a guidewire having visualizable markers separated by a known distance can be inserted into the treatment region. However, these techniques have not become widespread in part because marker wires do not perform as well as the specialty guidewires preferred by physicians.
What is needed is an implant and associated delivery system that permits delivery and deployment of stents that are well matched to the length of diseased segments.